The present invention relates generally to light emitting devices using compound semiconductor materials. More particularly, the present invention relates to high power, high luminous flux light emitting diodes.
Light emitting diode (LED) technology has revolutionized lighting equipment in recent years. Due to the advantages offered by light emitting diodes (LEDs), many applications now incorporate LEDs instead of conventional incandescent lighting sources. These applications include, but are not limited to, traffic signaling, electronic signs, medical applications, instrumentation, and general illumination. LEDs generally consume much less power as equally luminous incandescent lamps, and LEDs are also much more durable than conventional incandescent lighting sources. This leads to less frequent replacements and lower maintenance costs. Also, less electrical power consumption by the LEDs translates into less strain on a power source, such as an alternator or battery. LEDs are also insensitive to vibration and have lower switch-on time in comparison to most incandescent lighting sources.
For LEDs to replace incandescent lighting sources in applications as described above, the LEDs will have to provide high luminous output while maintaining reliability, low power consumption and low manufacturing cost. In many of the above-described applications, the LEDs are in the form of LED chips having an edge length of around 300 xcexcm. An individual LED chip of this type usually has low power output and can only be subjected to low injection current. As a result, these LED chips need to be assembled into clusters or arrays to achieve the required luminous flux level.
Multiple clusters or arrays of LED chips are generally mounted onto a board and then integrated with a lamp housing, electronics, and various lenses. Due to the small size of these LED chips and the limited amount of luminous flux that each can generate, the number of LED chips necessary to achieved the required flux levels is generally quite large. This increases the complexity in packaging and installing LED chips for a particular application, in terms of both time and manufacturing cost. For example, much time and manufacturing cost are needed for mounting, optical collecting, and focusing the emissions from the LED chips. Extra time and cost are also required to install and aggregate the LED chips in a specific arrangement as required by a specific application.
Attempts have been made to manufacture LED chips that are capable of creating higher luminous flux than the xcx9c300 xcexcm edge length LED chips. One approach is to increase the edge length and make each LED chip larger. The larger size allows more current to flow over and through the LED chip, and higher luminous flux is generated per LED chip as a result. Although the larger size simplifies packaging and installation of the LED chips because a fewer devices are required to be packaged and installed, reliability and power consumption become problematic. Specifically, larger size LED chips currently available are limited in their power and luminous flux output. For example, several commercial devices currently available are limited to a current dissipation of approximately 350 mA.
The primary limiting factor in larger LED chips is the inability for current to spread evenly over and through the entire structure of an LED chip. Rather, the current accumulates at specific spots on the LED chip, preventing the efficient use of the available lightemitting semi-conductive material. This phenomenon is commonly referred to as xe2x80x9ccurrent crowding.xe2x80x9d Current crowding tends to occur at points on electrical contacts of an LED chip because of the tendency of charge carriers to travel a path of least resistance. Current crowding may also occur in certain regions of the electrical contacts depending on the capacity for each of the regions to accept and spread current. Current crowding leads to unstable luminous flux output with bright spots and dim spots on the LED chip. Current crowding also necessitates more current to be injected into the LED chip, which leads to high power consumption and can cause breakdown in the LED chip. As a result, light is not emitted efficiently, and power consumption is not minimized. Moreover, the larger size LED chips currently available include additional limiting factors that further contribute to its limited power and limited luminous flux output. These limiting factors include ineffective heat dissipation, deficient light enhancing structure, and limited number of light emitting regions that results in high light re-absorption within the device structure. Therefore, high power, high luminous flux LED chips cannot be achieved using conventional means.
Aspects of the present invention relate to high power, high luminous flux light emitting diodes and the methods of making them. In one embodiment, the light-emitting diode comprises a substrate, a light-emitting structure disposed above the substrate along a vertical axis, a P electrode having a number of legs extending in one direction along a substantially horizontal axis perpendicular to the vertical axis, and an N electrode having a number of legs extending substantially horizontally in the direction opposite to the direction of the legs of the P electrode. The light-emitting structure includes a P cladding layer, an active layer and an N cladding layer. The P electrode is in contact with the P cladding layer of the light-emitting structure, while the N electrode is in contact with the N cladding layer of the light-emitting structure. The N electrode is disposed at a lower surface than the P electrode, where the lower surface is defined by a mesa etch process, forming a mesa edge separating the N electrode from the P electrode. A thin metal layer is under the P electrode, which is overlapped and in contact with the P electrode and separated from the N electrode by the mesa edge. The P and N electrodes are designed in such a manner that portions of the legs of the P electrode are interspersed with and spaced apart from portions of the legs of the N electrode.